A high speed turbo machine, such as, for example, a steam or gas turbine, generally comprises a plurality of blades arranged in axially oriented rows, the rows of blades being rotated in response to the force of a high pressure fluid flowing axially through the machine. It is common to monitor the position of the blades relative to a flowpath wall within the turbine, both during the design and testing of the turbine and during normal operation of the turbine. For example, it is known to use non-contacting proximity sensors or probes to detect a gap distance between the blade tips and the flowpath wall, as well as detect blade vibrations.
One conventional proximity sensor includes a capacitance gap sensor that has a single sensing electrode that is energized by a voltage so as to generate an electric field in the expected path of a turbine blade. The casing of the turbine provides a virtual ground for the electrode such that the electrode and the turbine casing act as a capacitor. When a turbine blade passes through the generated electric field, the capacitance between the electrode and the turbine casing changes as well. A change in the electrode's energizing voltage may be detected as a result of a change in the capacitance between the electrode and the virtual ground. The magnitude of the change in the energizing voltage is used as an indicator of a proximity of the turbine blade to the electrode.
The above approach has a number of drawbacks. The noise resulting from using the turbine casing as the virtual ground reduces the distance which the sensor can be separated from the circuitry which generates the energizing voltage and analyzes the results. Calibrating a detected change to a predetermined distance is difficult and is limited to a specific installation because the composition and the shape of the turbine blades have an effect on the magnitude at which an energizing voltage may change. Additionally, the ambient conditions where the sensor is located affects the magnitude of a resulting change in the sensor's capacitance. Furthermore, the conditions within a turbine, such as near the first and second row, may reach temperatures of about 1700 C or more. Operation in such an environment can degrade the performance of a conventional capacitance gap sensor such that it may fall out of calibration in a matter of days or weeks.
Accordingly, there is currently an unmet need for a proximity sensor, for example a turbine blade proximity sensor, which provides accurate results in a variety of environments, over a relatively long period of time without re-calibration.